1. Field of the Invention
The present invention relates to a laminated ceramic electronic component and a method for manufacturing the laminated ceramic electronic component, and more particularly, to a laminated ceramic electronic component with external terminal electrodes formed directly by plating so as to be electrically connected to a plurality of internal electrodes, and a method for manufacturing the laminated ceramic electronic component.
2. Description of the Related Art
As shown in FIG. 4, a laminated ceramic electronic component 101 typified by a laminated ceramic capacitor generally includes a component main body 105 having a laminated structure, which includes a plurality of laminated ceramic layers 102 made of, for example, a dielectric ceramic, and a plurality of layered internal electrodes 103 and 104 arranged at interfaces between the adjacent ceramic layers 102. Respective ends of the plurality of internal electrodes 103 and respective ends of the plurality of internal electrodes 104 are exposed at opposed end surfaces 106 and 107 of the component main body 105, and external terminal electrodes 108 and 109 are respectively arranged so as to electrically connect the respective ends of the internal electrodes 103 to each other and the respective ends of the internal electrodes 104 to each other.
For the formation of the external terminal electrodes 108 and 109, typically a metal paste including a metal component and a glass component is applied onto the end surfaces 106 and 107 of the component main body 105, and then baked, thereby first forming paste electrode layers 110. Next, first plating layers 111 primarily including, for example, nickel are formed on the paste electrode layers 110, respectively, and second plating layers 112 primarily including, for example, tin or gold are further formed thereon. Thus, each of the external terminal electrodes 108 and 109 is formed in a three-layer structure including the paste electrode layer 110, the first plating layer 111, and the second plating layer 112.
The external terminal electrodes 108 and 109 are required to have excellent solderability when the laminated ceramic electronic component 101 is mounted on a substrate using solder. At the same time, the external terminal electrode 108 is required to electrically connect the plurality of internal electrodes 103 which are electrically insulated from each other to each other, and the external terminal electrode 109 is required to electrically connect the plurality of internal electrodes 104 which are electrically insulated from each other to each other. The second plating layers 112 described above ensures the solderability, whereas the paste electrode layers 110 electrically connect the internal electrodes 103 to each other and the internal electrodes 104 to each other. The first plating layers 111 prevent solder erosion in the solder joint.
However, the paste electrode layer 110 has a large thickness from several tens of μm to several hundreds of μm. Therefore, in order to limit the dimensions of the laminated ceramic electronic component 101 within certain specifications, there is undesirably a need to reduce the effective volume for ensuring a capacitance because there is a need to ensure the volumes of the paste electrode layers 110. On the other hand, the plating layers 111 and 112 have a thickness on the order of several μm. Thus, if the external terminal electrodes 108 and 109 can be defined only by the first plating layers 111 and the second plating layers 112, the effective volume for ensuring the capacitance can be increased.
For example, Japanese Unexamined Patent Publication No. 63-169014 discloses a method in which a conductive metal film is deposited by electroless plating on the entire sidewall surface of a component main body, at which internal electrodes are exposed, so as to short circuit the internal electrodes exposed at the sidewall surface.
However, the method for forming an external terminal electrode described in Japanese Unexamined Patent Publication No. 63-169014 involves no paste electrode layer formed by backing, and thus has a problem in that a plating solution and other moisture are likely to enter the component main body along the interfaces between the internal electrodes and ceramic layers. Therefore, when plating is applied directly to exposed ends of the internal electrodes, the plating solution may enter the component main body along the interfaces between the internal electrodes and the ceramic layers, and corrode the ceramic defining the ceramic layers and the internal electrodes, thereby causing structural defects. Furthermore, this also results defects in terms of reliability, such as degraded load characteristics against humidity for the laminated ceramic electronic component.
In particular, when tin or gold plating is to be applied, the problems described above are more likely to occur because tin and gold plating solutions usually contain a highly corrosive complexing agent.
To overcome the problems described above, International Publication No. WO2008/059666 discloses forming a plating layer defining an external terminal electrode directly on an end surface of a component main body, and then forming an interdiffusion layer at a boundary section between the internal electrodes and the plating layer by performing a heat treatment. In the interdiffusion layer, cubical expansion of metal occurs. Thus, gaps which may be present at the interfaces between the ceramic layers and each of the internal electrodes and external terminal electrode can be advantageously filled.
Furthermore, when the technique described in International Publication No. WO2008/059666 is used, the fixing strength at the interface between the ceramic defining the ceramic layers laminated with the internal electrodes interposed therebetween and the plating layer is also improved in the component main body. Thus, when the improvement of the fixing strength is required, it is preferable to perform a heat treatment at a temperature of 1000° C. or more, which is the eutectic temperature of metal defining the plating layer.
However, in the heat treatment at a temperature of 1000° C. or more as described above, when the temperature is linearly increased until it reaches the temperature, voids may occur due to a difference in diffusion velocity between the metal defining the plating layer and the metal defining the internal electrodes. In particular, such voids will occur in the metal having a higher diffusion velocity. For example, in the case of a combination of the internal electrodes primarily including nickel and the plating layer primarily including copper, voids will occur in the copper plating layer.
The occurrence of such voids not only decreases the adhesion strength between the external terminal electrode and the internal electrodes, but also decreases the reliability, such as load characteristics against humidity, of the laminated ceramic electronic component.